Interphotoreceptor Retinoil-Binding Protein (IRBP) was purified to apparent homogeneity from macaque monkey and bodine interphotoreceptor matrix (IPM). The amino acid compositions of the monkey and bovine proteins were similar, with non-polar amino acids comprising more than 50 percent of the residues. Amino termial analysis showed considerable homology between monkey and bovine IRBPs in this region. Monkey IRBP had an apparent molecular weight of 106,000 + 2900 on sedimentation equilibrium ultracentrifugaton. Both monkey and bovine IRBPs had isoelectric points between 6 and 7. Immunofluorescence studies using affinity purified rabbit anti-monkey IRBP demonstrated the resence of IRBP in Y-79 human retinoblastoma cells grown in monolayer tissue culture. The secretion of IRBP by monkey retina in short-term organ culture was blocked by monensin but was not affected by tunicamiycin or swainsonine.